This is the third oldschool card back brought back to the shop by Blizzard, after Naxxramas card back and Cupcake card back over the last few months. And now another one is available (thus we assume that more “popular” card backs will be back in the coming months).
Originally, Magic of Dalaran card back was awarded for achieving Rank 20 during Ranked Season 39 (June 2017). So if you’re a newer player, or you have just missed that specific season, you can get it now.
The card back is available for either 500 Gold or $2.99 / €2.99. It’s a limited time offer, but we don’t know how long will it last – most likely around 1-2 weeks.